5.4- DOUBLE SLIT INTERFERENCE

Question 1

How can the interference of light be observed?

Answer 1

illuminate two closely spaces parallel slits (double slits) using a suitable light source

Question 2

What do the two slits act as in the double slit experiment?

Answer 2

act as coherent sources of waves

Question 3

What does it mean for wave to be coherent?

Answer 3

emit light waves with constant phase difference and same frequency

Question 4

What would have Young used instead of a light bulb to illuminate a narrow single slit in his double slit experiment?

Answer 4

would have used a candle

Question 5

What is the double slit arrangement illuminated by?

Answer 5

light from narrow single slit

Question 6

What can be seen on the white screen from the double slit experiment?

Answer 6

alternate bright and dark fringes, referred to as Young’s fringes

Question 7

Why can Young’s fringes be seen on the white screen from the double slit experiment?

Answer 7

this is where diffracted light from the double slits overlap

Question 8

How are the fringes spaces from the double slit experiment?

Answer 8

evenly spaced and parallel to the double slits

Question 9

What happens if the single slit is too wide in the double slit experiment?

Answer 9

each part of it produces a fringe pattern, which is displayed slightly from the pattern due to adjacent parts of a single slit

Question 10

What happens to the fringes when the slit is too wide in the double slit experiment?

Answer 10

dark fringes of double slit pattern becomes narrower than bright fringes, + contrast lost between dark and bright fringes

Question 11

What can be used instead of a light bulb and a single slit for the double slit experiment?

Answer 11

a laser beam

Question 12

Why must the fringes be displayed on a screen when a beam of laser light is used in the double slit experiment?

Answer 12

beam of laser light will damage the retina if it enters the eye

Question 13

What are the fringes formed due to?

Answer 13

interference of light from the two slits

Question 14

How are the bright fringes formed?

Answer 14

light from one slit reinforces light from other slit

light waves from each slit arrives in phase with each other

Question 15

How are the dark fringes formed?

Answer 15

light from one slit cancels light from other slit

light waves from two slits arrive 180° out of phase

Question 16

What is the fringe separation?

Answer 16

distance from centre of a bright fringe to centre of next bright fringe

Question 17

What is the equation for fringe separation?

Answer 17

w = λD / s

Question 18

How can the fringes become more widely spaced? (distance)

Answer 18

distance D from slits to screen increase

Question 19

How can the fringes become more widely spaced? (wavelength)

Answer 19

wavelength λ of light used increased

Question 20

How can the fringes become more widely spaced? (slit spacing)

Answer 20

slit spacing s reduced

Question 21

What is the slit spacing?

Answer 21

distance between centres of slits

Question 22

When only is the equation w = λD / s valid?

Answer 22

only if fringe separation much less than distance D from slits to screen

Question 23

How would you measure the fringe separation?

Answer 23

measure across several fringes from centre of dark fringe to centre of another dark fringe
then divide measurements by number of fringes you measured across

Question 24

Why would you use the dark fringes when measuring the fringe separation?

Answer 24

as centres of dark fringes easier to locate than centres of bright fringes

Question 25

What can demonstrate interference?

Answer 25

two loudspeakers connected to same signal generator

Question 26

How can two loudspeakers connected to the same signal generator be used to demonstrate intereference?

Answer 26

as they’re coherent sources of sound waves

Question 27

How can you detect points of cancellation + reinforcement with two loudspeakers connected to the same signal generator?

Answer 27

by ears as you move across in front of the speakers

Question 28

What can be used to estimate the wavelength of sound waves (provided wavelength small compared with distance between speakers)?

Answer 28

young’s slit equation